The invention relates to a high-pressure discharge lamp provided with a discharge vessel which is enclosed with intervening space by an outer bulb fitted with a lamp cap, and provided with an ignition circuit in which a fuse is included, which discharge vessel is provided with electrodes between which a discharge extends in the operational condition of the lamp, each electrode being connected to a relevant current supply conductor.
A lamp of the kind mentioned in the opening paragraph is known from EP-A-431,696 which corresponds to U.S. Pat. No. 5,055,676. In the known lamp, the ignition circuit is suitable for being operated in series with a stabilizer ballast on an AC-voltage supply source and is arranged in the outer bulb. The known lamp is a high-pressure sodium lamp which is suitable for operation in an installation designed for a high-pressure mercury discharge lamp. The relevant stabilizer ballast is in general not protected against loads in the form of short-circuit currents through the lamp, more particularly through its ignition circuit. This is why a fuse is included in the ignition circuit, which fuse melts in the case of a short-circuit in the ignition circuit, thus protecting the stabilizer ballast against excessively high currents.
The fuse is positioned in vacuum. Preferably, the space enclosed by the outer bulb is evacuated because this is favourable for the luminous efficacy of the lamp.
It was found in practice that melting of the fuse in the known lamp, which depends primarily on the diameter of the wire from which the fuse is manufactured, requires a comparatively high current, and also that the time during which this current is carried before actual melting takes place varies very strongly. Especially the spread in fusion time is a serious drawback, since it detracts strongly from the reliability of the fuse.